1. Field of the Invention
The present invention relates to a method of treating sewage or waste water and apparatus therefor, particularly, for removing nitrogen and phosphorus as well as organic materials in the sewage or waste water, by applying intermittent aeration and dynamic flow to an equipment having at least a pair of reaction basins with intermittent aeration means, a clarifier with sludge transfer means, and a filter for passing liquid without passing solid between the reaction basins.
2. Description of the Related Art
Biological method of removing nitrogen and phosphorus in a waste water treatment plant includes an anoxic process in which free oxygen is not supplied, an anaerobic process, and an aerobic process in which oxygen is supplied. In an aerobic reaction process, organic nitrogen and ammoniacal nitrogen are oxidized into nitrate. In an anoxic reaction process, denitrification process in which the nitrate is reduced into nitrogen gas to then be released to the air is carried out. In an anaerobic reaction process, phosphorus is released from activated sludge. The released phosphorus components are taken up luxuriously by microorganisms in an aerobic reaction process. Further, the microorganisms having done the luxury uptake are removed by the waste sludge, thereby finally removing phosphorus. That is, nitrogen and phosphorus are removed by the successive repetition of anaerobic-anoxic-aerobic process.
In a conventional method for removing nitrogen and phosphorus, anaerobic basins, anoxic basins and aerobic basins are separately equipped with a constant capacity. Thus, it is impossible to meet with changes in quality and amount of influent waste water with flexibility. Also, methanol must be injected into the plant as an electron donor for denitrification, or water in a nitrification basin must be circulated to the denitrification basin of the previous phase to utilize organisms contained in the waste water. In case of injecting methanol, it costs a lot for chemicals, and in case of utilizing the organisms, the circulating flow is about 3xcx9c4 times of the influent, which increases the cost of maintenance and management including pump facilities and power supply.
To overcome such problems, methods of intermittent aeration and flow path change have been proposed. As typical conventional arts employing the intermittent aeration and flow path change, there is so called PID (Phased Isolation Ditch).
FIGS. 8A and 8B show schematically the mechanism of action of a conventional PID process for removing nitrogen and phosphorus, illustrating flow path changes in an aerated or an-aerated state for the respective phases A through D.
The overall configuration of equipment for the PID process will be described in the order of progress hereinbelow. In view of the progress of inflow, the equipment is constructed by a preliminary denitrification basin 201a, a selection basin 201b, an anaerobic basin 201c, at least two sets of oxidation ditches 202 and 203 each having an aerator and a mixer, and a clarifier 204 having a sludge collector 206. Also, there are equipped with a sludge return pump 205 and a sludge return pipe 208 for returning the sludge from the clarifier 204 to the preliminary denitrification basin 201a. 
The function of the anaerobic basin 201c is to mix raw waste water with the sludge returned from the clarifier 204 and to release the phosphorus from the sludge while an anaerobic state is maintained. At this time, if chemically combined oxygen such as nitrate (NO3) or nitrite (NO2) exists, phosphorus is hardly released from the sludge. Thus, in the previous phase of the anaerobic basin 201c, free oxygen or nitrate contained in the raw waste water or returned sludge is first removed in the preliminary denitrification basin 201a and the selection basin 201b. The anaerobic basin 201c is constructed with at least two sets of basins in combination to prevent short circuit, and each reaction basin is equipped with a mixer 301.
The clarifier 204 is an external equipment independently installed outside the oxidation ditches 202 and 203, and the sludge collector 206, the sludge return pump 205 and the return pipe 208 are additionally installed therein. Here, the sludge return flow must be more than the total amount of the inflow.
As described above, in view of facilities, maintenance and management, the PID process requires much cost of installation, electricity and facilities management due to the construction of the preliminary denitrification basin, the selection basin, the anaerobic basin and the clarifier. Further, in the PID process, since a change between phases is not fast and clear, the processing efficiency may be lowered. The activated sludge having a reduced content of phosphorus by the release of phosphorus in an anaerobic state is converted to take up phosphorus luxuriously in an aerobic state since the microorganisms are activated. In the PID process, however, the sludge passed through the release of phosphorus in the anaerobic basin is introduced to an anoxic state in phases A and C, not to an aerobic state. Thus, the microorganisms may not be sufficiently activated, which lowers the efficiency of phosphorus uptake.
During a denitrification process, sufficient amount of organisms are required as electron donors to reduce nitrogen oxide, In the PID process, however, the sludge on which a large amount of organisms required for denitrification are adsorbed is continuously released from the oxidation ditch in which denitrification is carried out in an anoxic state, and the sludge is introduced into the oxidation ditch where a nitrification process is carried out and a large amount of organisms is unfavorable. Hence, the nitrification process requires much more time and the denitrification efficiency in the anoxic oxidation ditch is lowered for lack of organisms. That is, in the phase A in PID process, the same amount of sludge as that of inflow is continuously released from the first oxidation ditch 202 where denitrification is carried out, then flows into the second oxidation ditch 203 where nitrification is carried out. Thus, the organisms adsorbed in the sludge are washed away from the first oxidation ditch 202, which is unfavorable to denitrification. Then, the organisms are introduced into the second oxidation ditch 203 where nitrification is carried out, which is also unfavorable to nitrification. These situations also occur in phase C in which the flow path is changed and denitrification is carried out in the second oxidation ditch 203.
To solve the above problems in the conventional PID process, it is an object of the present invention to provide a method of treating sewage or waste water for removing efficiently nitrogen and phosphorus by applying intermittent aeration and change of flow path, which curtails the cost of equipment and maintenance of facilities.
It is another object of the present invention to provide an apparatus for the treatment method as described above.
To accomplish the above object, the present invention provides a method of treating a waste water for removing nitrogen and phosphorus comprising the following steps carried out repeatedly in a system comprising at least a pair of reaction basins with an intermittent aeration means, a clarifier for precipitating the reacted waste water from the reaction basins, and a filtering means equipped between the reaction basins:
introducing a raw waste water into a first reaction basin where denitrification and release of phosphorus are carried out in an anaerobic condition, discharging the waste water through the filtering means into a second reaction basin where nitrification and decomposition of organisms are carried out in an aerobic condition, and discharging the treated waste water through the clarifier and returning a sludge into the second reaction basin (phase A);
introducing a raw waste water into the second reaction basin where an aerobic reaction is carried out in an aerobic condition, and discharging the treated waste water through the clarifier and returning a sludge into the second reaction basin, while in the first reaction basin, removing phosphorus through luxury uptake of the phosphorus into the sludge in an aerobic condition without inflow and outflow (phase A-1)
introducing a raw waste water into the second reaction basin where denitrification and release of phosphorus are carried out in an anaerobic condition, discharging the waste water through the filtering means into the first reaction basin where nitrification and decomposition of organisms are carried out in an aerobic condition, and discharging the treated waste water through the clarifier and returning a sludge into the first reaction basin (phase B); and
introducing a raw waste water into the first reaction basin where an aerobic reaction is carried out in an aerobic condition, and discharging the treated waste water through the clarifier and returning a sludge into the first reaction basin, while in the second reaction basin, removing phosphorus through luxury uptake of the phosphorus into the sludge in an aerobic condition without inflow and outflow (phase B-1).
To accomplish another object of the present invention, it is provided an apparatus for treating a waste water for removing nitrogen and phosphorus comprising:
at least a pair of reaction basins equipped with an intermittent aeration means;
a filtering means for passing the waste water between the reaction basins;
a clarifier for precipitating the waste water from the reaction basins;
a first flow path for introducing a raw waste water;
a second flow path for introducing the waste water from the first flow path into each reaction basin separately;
a third flow path for discharging the waste water from each reaction basin separately;
a fourth flow path for introducing the waste water from the third flow path into the clarifier;
a fifth flow path for discharging the waste water from the clarifier;
a sixth flow path for introducing a sludge into each reaction basin separately; and
flow path control means equipped on the flow paths.
According to the present invention, in the reaction basin requiring an anaerobic or anoxic condition, outflow of organisms and inflow of free oxygen or nitrogen oxides from the reaction basins where decomposition of organisms and nitrification are carried out in an aerobic condition are prevented, which enhances the efficiency of denitrification. Further, in the reaction basin where nitrification is carried out in an aerobic condition, inflow of organisms from the reaction basin in an anaerobic or anoxic condition is prevented, which enhances the efficiency of nitrification. That is, the transfer of sludge is prevented between the two reaction basins in which different kinds of reactions are carried out each other, which improves both the reactions carried out in each of the reaction basins.
Further, the present invention introduce a system having flow paths and means for controlling the flow paths where the direction of flow of waste water and the direction of inflow of returned sludge are varied, for example, returned sludge is introduced into the reaction basin from which reacted waste water is discharged into a clarifier.